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(11) | EP 0 423 987 A3 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Apparatus and method for extracting the RMS value from a signal |
| (57) Disclosed is apparatus and method for converting and displaying the total RMS and
AC RMS values of a signal in a voltmeter. This conversion is accomplished in a system
having a processor (108) and a plurality of signal conversion stages (104). The first
signal conversion stage comprises a low-pass filter (202) to extract the DC component
of the input signal followed by an analog-to-digital converter (206) to convert the
DC component into a digital value. The remaining signal conversion stages convert
the AC component of the input signal to a series of smaller and smaller DC average
values. Typically there are five of these AC conversion stages, each comprising a
high-pass filter (210) connected to a full-wave rectifier (214) which is in turn connected
to a low-pass filter (218) followed by an analog-to-digital converter (222). The first
AC stage high-pass filter (210) is connected to the input signal (102) and subsequent
AC stage high-pass filters are connected to the output of the full-wave rectifier
of the previous AC conversion stage. A processor (108) inputs all the digital values
from the various conversion stages, squares each of the values, and computes the sum
of these squares. The processor (108) then computes the square root of this sum to
provide the RMS or AC RMS value. If the DC component digital value is used in the
computation the result is the total RMS voltage of the input, but if the DC component
is excluded, the result is the AC RMS value. The processor (108) then displays either
the totai RMS or the AC RMS value on the display (112) of the voltmeter. |